1. Field of the Invention
This invention relates to a refrigerant evaporator, which is suitable for an automotive air conditioner and includes plural flat refrigerant passages holding inner fins therein.
2. Description of the Related Art
A conventional refrigerant evaporator for an automotive air conditioner provides therein tube-like refrigerant passages defined by metallic thin plates laminated with one another and holding inner fins for increasing heat transfer areas on a refrigerant side. When the inner fins are bent to meander in a cross-section, the refrigerant passages are respectively divided by the meandering inner fins into several straight tube-like sub-passages in which refrigerant independently flows.
The inventor of the present invention has studied and examined this kind of refrigerant evaporator in which several sub-passages are independently provided in the respective refrigerant passages. As a result, it was found that the following phenomenon occurred to adversely affect evaporator characteristics.
That is, because the sub-passages divided by the meandering inner fins are completely independent in the evaporator, the refrigerant flowing in one of the sub-passages is not mixed with refrigerant flowing in the other sub-passages from an inlet to an outlet of the refrigerant passages. Therefore, when there arises variation (under and over supplies) in refrigerant distribution at the inlet of the refrigerant passages, the variation is kept until refrigerant flows out from the outlet.
In ordinary operational conditions of the evaporator, a volume of gaseous refrigerant is approximately 70 times as large as that of liquid refrigerant. Therefore, a flow resistance of gaseous refrigerant transformed from liquid refrigerant is significantly increased. When a gaseous refrigerant region is large, it becomes difficult for refrigerant to flow. In addition, when a refrigerant amount is short relative to a heat load on an air side in one of the sub-passages, refrigerant starts to evaporate at an upstream side more than that of the other sub-passages in which the refrigerant amount is not short, so that the gaseous refrigerant region in the one of the sub-passages is increased. This additionally promotes the refrigerant shortage caused by the under-supply.
On the other hand, in another one of the sub-passages in which the refrigerant amount is large, refrigerant starts to evaporate at a downstream side more than that in which the refrigerant amount is short. Therefore, the gaseous refrigerant region is relatively decreased so that refrigerant becomes liable to flow. As a result, the refrigerant excess caused by the over-supply is further promoted. That is, the under and over supplies of refrigerant into the sub-passages when the evaporator starts to be operated are further promoted during the heat exchange between refrigerant and air. Consequently, evaporator cooling property is prominently lowered as compared to an ideal state where evaporation (heat exchange) of refrigerant is uniformly performed in all of the sub-passages.
Further, temperature of air flowing in a heat exchanging part of the evaporator is lowered as air flows from an air flow upstream side to an air flow downstream side. Therefore, it is inevitable that an optimum refrigerant amount in the sub-passage provided at the air flow downstream side is smaller than that in the sub-passage provided at the air flow upstream side. Because of this, even when refrigerant is uniformly distributed into the sub-passages, refrigerant shortage inevitably occurs in the sub-passage at the air flow upstream side and refrigerant excess occurs in the sub-passage at the air flow downstream side.
To solve this problem, JP-U-59-76886 proposes a heat exchanger including a flat refrigerant passage, in which a corrugated inner fin formed with many louvers allowing refrigerant to flow from an air flow upstream side to an air flow downstream side is disposed therein. Accordingly, refrigerant shortage in a sub-passage at the air flow upstream side is relieved.
As a result of experimental studies on the above-describe heat exchanger by the inventor, however, it is founded that because the louvers are arranged on the entire area of the inner fin, pressure loss on the refrigerant side is increased by the louvers so that a refrigerant evaporation pressure in the evaporator is increased, resulting in decreased difference between refrigerant evaporation temperature and air temperature. Because of this, the cooling property of the heat exchanger adopting the louvers cannot be desirably improved, even when the number of the louvers is increased.